This has happened to me a couple of times so far. It only happens after I add a bunch of static meshes and then run the PIE. Everyone blows up except me and the FPS drops from 45-50 fps to 5 fps. I've tried to fix it by re-building everything and/or closing the editor and re-starting it. Generally this fixes the problem but I always come back to a lower frame rate. I thought it might be memory related and have shrunken the LightmassImportanceVolume but I still get lower frame rates than I get on a full 64-person server. I've followed Yoshiro's recommendations on performance settings for the terrain but haven't messed with LOD's for my static meshes.

Just to give more detail, I only have about 300 objects (static meshes, destructables and 1 dominant directional light) in the map right now and terrain measures 64 x 64 x 256

Here's a couple of shots of the pylons as they relate to my spawns. What do you think? The first shot (in the field) is the German spawn and the one in the middle of the houses is the Soviet spawn. I've had the blowing-up problems in both spots.

Cat check your spawn volume is not floating above the playerstart thingys.
Occasionally mine have moved and are not encompassing the playerstarts - the bots all die at spawn and frame rate turns to hell.

You can use Stat Engine, Stat Memory, Stat Units to get an idea what is chewing up the cycles. Just look for things with unusual amounts of time. 10ms is high in things outside of script, game and render counters. There are probably other +10ms cycles but you can sorta baseline off of a similar sized stock map.

As to the destructible BOTs, make sure you have at least one cover node in the objective volume. Normally when there is a cult like suicide, as you describe, it's because the AI entities cannot path to the objective or cannot find any cover in the objective space.

Cat check your spawn volume is not floating above the playerstart thingys.
Occasionally mine have moved and are not encompassing the playerstarts - the bots all die at spawn and frame rate turns to hell.

It turned out that when I was placing static meshes I moved the terrain just enough to mess up a couple of the pylons, including pylons over both the Allied and Axis spawns. I re-adjusted the terrain and moved the pylons a bit then re-built the whole mess. Everything works fine now and my frame rate is back up in the 40's. I'm going to folow TWSwag's suggestions as well to track down some performance issues as well.